Multiple orifice valves are useful for control of flow of fluids such as fluids produced from oil and gas wells and the like. Orifice valves, for example, are used on a wellhead for control of the rate of flow of well production fluids from a well. Such valves are often referred to as chokes. Exemplary multiple orifice valves are described in U.S. Pat. Nos. 3,207,181 and 3,331,396 by R. S. Willis, both of which are incorporated by this reference.
The rate of flow of fluid through an orifice valve is in part determined by the number and size of holes in both a stationary disk and a rotatable disk mounted across a fluid path through a valve body. In an exemplary valve, there are two diametrically opposite holes in each of the upstream and downstream disks. The holes in the rotatable upstream disk are positioned in various degrees of alignment with the holes in the stationary downstream disk by angular movement of the rotatable disk. The rotatable disk can be moved from a fully closed position out of alignment with the holes in the downstream disk for blocking flow through the valve to a fully opened position with the holes aligned for providing maximum flow through the valve.
Such a valve or choke is used for controlling the rate of production from an oil well by rotating the upstream disk to a position at an angle of up to 90.degree. from the closed position. At some position within this range 20%, for example, of the cross-sectional area of each of the holes in the upstream disk overlaps 20% of the area of the holes in the downstream. The balance of each of the holes is occulted by the other disk. When in such a position the flow through the valve is 20% of the flow when the valve is fully open. This percentage and others can be indicated by indicia on the valve stem so that the valve can be set to a desired flow rate.
It is desirable to reliably adjust such a valve to reproducible valve openings. This is difficult at low flow rates, however, since the percentage change in flow is rather large for a small angular displacement of the disk. Reproducible adjustment can therefore be difficult.
Setting of an adjustable choke on an oil or gas well is quite often near the low end of the range of flow through the valve. This occurs because of the conservative design employed in the oil patch. A larger valve than might appear necessary is commonly used at the wellhead so that the valve can be left in place for a long time during the productive life of the well. Such a valve can be set with a small valve opening to obtain a desired production flow during early life of a well when production pressures are high. Thereafter, as production pressure gradually decreases, the valve can be gradually opened for maintaining a desired flow. Such a valve may be used in the low ranges of flow control, that is less than half open, for periods of months or years.
Multiple orifice valves have a notoriously tight calibration range at low flow rates, particularly up to about 20% of maximum flow, and it is desirable to extend the calibration range that can be used reliably and reroducibly.